A water analysis measurement arrangement for determining ions or ionic compounds solvated in water, in particular for determining the concentration of hydrogen ions, i.e., the so-called pH, is known from the prior art. Such systems, e.g., for determining pH, function according to the principle of electrochemical cells that are typically designed as glass electrodes or so-called combination electrodes.
A conventional electrochemical cell comprises two separate half-elements that each accommodate an electrode, one of the half-elements containing a measuring electrode and the other half-element containing a reference electrode. Both half-elements are in separate solutions which are connected through an electrolyte bridge. As an alternative, both half-elements may also be immersed in a common solution. The two half-elements together form a measuring chain.
Regarding the structure, an electrochemical cell of this type is often realized as a double-walled glass tube. The inner tube contains a measuring electrode. The inner tube also has a semi-permeable glass membrane at the end thereof which establishes the contact to the measurement solution. The outer tube accommodates a reference electrode in contact with the measurement solution through an electrolyte bridge, such as a double diaphragm or a single diaphragm. Both tubes are usually filled with a neutral buffered electrolyte solution with a pH of 7.
What is measured is the electrical voltage between the reference electrode and the measurement electrode which itself depends on the hydrogen ion concentration in the measurement solution.
This design of a combination electrode has a weak point in the diaphragm that establishes an electric contact between the reference electrode and the measurement solution. Depending on the concentration of the ions present in the measurement solution, different chemical potentials between the reference electrode chamber and the measurement electrode may gradually cause a contamination and/or a dilution of the buffered electrolyte solution in the reference electrode chamber. Such a contamination and/or dilution have the effect that the pH indicated does not reflect the actual hydrogen ion concentration. If the contamination and/or dilution is too marked, the existing electrolyte solution must also be replaced which requires substantial effort. A soiling and/or contamination of the diaphragm facing the measurement solution may also occur, thereby resulting in significant erroneous potentials.
When used in an industrial control process, such a design has a further problem, that being the electrical noise of the environment in which the pH system is installed. This results in measurement inaccuracies that may have serious consequences in an industrial control process.
A water analysis measurement arrangement described in US 6 395 158 B1 solves the problem of electrical noise. A third electrode, the so-called ground rod, is here provided in addition to the known dual-electrode arrangement, wherein a measurement of the differential between the measurement electrode and the reference electrode is performed with respect to the third electrode, i.e., the ground rod.
A water analysis measurement arrangement is described in WO 2007 023031 A1 which overcomes the weakness of the electrolyte bridge. The individual diaphragm is here replaced with a chamber including two individual diaphragms, the chamber being filled with an electrolyte. Due to this arrangement, the electrolyte solution in the reference electrode chamber can no longer be contaminated or diluted to the extent found in conventional arrangements. However, it is not possible to provide perfect protection of the electrolyte bridge in an attempt to avoid contamination or dilution, since the electrolyte bridge would then lose the necessary permeability. The condition of the electrolyte bridge has a significant influence on the reference signal quality so that this represents a source of errors that may have adverse effects on a control process.